It is known that a bulk acoustic-wave (BAW) device is, in general, comprised of a piezoelectric layer sandwiched between two electronically conductive layers that serve as electrodes. When a radio frequency (RF) signal is applied across the device, it produces a mechanical wave in the piezoelectric layer. The fundamental resonance occurs when the wavelength of the mechanical wave is about twice the thickness of the piezoelectric layer. Although the resonant frequency of a BAW device also depends on other factors, the thickness of the piezoelectric layer is the predominant factor in determining the resonant frequency. As the thickness of the piezoelectric layer is reduced, the resonance frequency is increased. BAW devices have traditionally been fabricated on sheets of quartz crystals. In general, it is difficult to achieve a device of high resonance frequency using this fabrication method. When fabricating BAW devices by depositing thin-film layers on passive substrate materials, one can extend the resonance frequency to the 0.5–10 GHz range. These types of BAW devices are commonly referred to as thin-film bulk acoustic resonators or FBARs. There are primarily two types of FBARs, namely, BAW resonators and stacked crystal filters (SCFs). An SCF usually has two or more piezoelectric layers and three or more electrodes, with some electrodes being grounded. The difference between these two types of devices lies mainly in their structure. FBARs are usually used in combination to produce passband or stopband filters. The combination of one series FBAR and one parallel, or shunt, FBAR makes up one section of the so-called ladder filter. The description of ladder filters can be found, for example, in Ella (U.S. Pat. No. 6,081,171, hereafter referred to as Ella '171). As disclosed in Ella '171, an FBAR-based device may have one or more protective layers commonly referred to as the passivation layers. A typical FBAR-based device is shown in FIGS. 1a to 1d. As shown in FIGS. 1a to 1d, the FBAR device comprises a substrate 501, a bottom electrode 507, a piezoelectric layer 509, and a top electrode 511. The electrodes and the piezoelectric layer form an acoustic resonator. The FBAR device may additionally include a membrane layer 505. As shown in FIG. 1a, an etched hole 503 is made on the substrate 501 to provide an air interface, separating the resonator from the substrate 501. Alternatively, an etched pit 502 is provided on the substrate 501, as shown in FIG. 1b. It is also possible to provide a sacrificial layer 506 separating the resonator and the substrate, as shown in FIG. 1c. It is also possible to form an acoustic mirror 521 between the bottom electrode 507 and the substrate 501 for reflecting the acoustic wave back to the resonator, as shown in FIG. 1d. The substrate can be made from silicon (Si), silicon dioxide (SiO2), Gallium Arsenide (GaAs), glass or ceramic materials. The bottom electrode and top electrode can be made from gold (Au), molybdenum (Mo), tungsten (W), copper (Cu), nickel (Ni), titanium (Ti), Niobium (Nb), silver (Ag), tantalum (Ta), cobalt (Co), aluminum (Al) or a combination of these metals, such as tungsten and aluminum. The piezoelectric layer 130 can be made from zinc oxide (ZnO), zinc sulfide (ZnS), aluminum nitride (AlN), lithium tantalate (LiTaO3) or other members of the so-called lead lanthanum zirconate titanate family. Additionally, a passivation layer typically made from a dielectric material, such as SiO2, Si3N4, or polyimide, is used to serve as an electrical insulator and to protect the piezoelectric layer. It should be noted that the sacrificial layer 506 in a bridge-type BAW device, as shown in FIG. 1c, is, in general, etched away in the final fabrication stages to create an air interface beneath the device. In a mirror-type BAW device, as shown in FIG. 1d, the acoustic mirror 521 consists of several layer pairs of high and low acoustic impedance materials, usually a quarter-wave thick. The bridge-type and the mirror-type BAW devices are known in the art.
It is also known in the art that FBARs can be used to form impedance element filters in a ladder filter configuration that has unbalanced input and output ports, or in a lattice filter configuration that has balanced ports. In some applications it would be advantageous to transform an unbalanced input to a balanced output (or vice versa) within a filter. Such filters have been produced using acoustically coupled surface acoustic wave (SAW) resonators. Basically these structures are based on a pair of resonators, as shown in FIG. 2. As shown, the first resonator 620 generates the acoustic wave and the second resonator 630 acts as a receiver. Since the resonators are not electrically connected, one of them can be connected as an unbalanced device and the other can be used in either as a balanced or an unbalanced device. As shown in FIG. 2, the first resonator 620 provides an unbalanced port 622 for signal input, whereas the second resonator 630 provides two ports 632, 634 for balanced signal outputs. As shown, numerals 610 and 640 denote reflectors or acoustic mirrors for the surface acoustic wave device. This same principle can be used in a BAW device having a structure that has two piezoelectric layers, one on top of each other. Using such a structure, it is possible to perform this unbalanced-to-balanced transformation. This structure can then be used as part of a filter or even a duplexer. One possible way of realizing such a structure is described in “High Performance Stacked Crystal Filters for GPS and Wide Bandwidth Applications”, K. M. Lakin, J. Belsick, J. F. McDonald, K. T. McCarron, IEEE 2001 Ultrasonics Symposium Paper 3E-6, Oct. 9, 2001 (hereafter referred to as Lakin). FIG. 3 is a coupled resonator filter (CRF) disclosed in Lakin. As shown in FIG. 3, the CRF is formed by a bottom electrode 507, a bottom piezoelectric layer 508, a cross-over electrode 511, a plurality of coupling layers 512, a ground electrode 513, a top piezoelectric layer 509 and two separate top electrodes 531 and 532. As such, the CRF has a first vertical pair 541 of resonators and a second vertical pair 542 of resonators. Each of the vertical pairs acts as a one-pole filter. In series, the two vertical pairs act as a two-pole filter. The CRF is made on a substrate 501 separated by an acoustic mirror 521.
Ella et al. (U.S. Pat. No. 6,670,866 B2, hereafter referred to as Ella '866) discloses a BAW device with two resonators and a dielectric layer therebetween. As shown in FIG. 4, the BAW device 20 is formed on a substrate 30 and comprises a first electrode 40, a first piezoelectric layer 42, a second electrode 44 connected to the device ground 12, a third electrode 60, a dielectric layer 50 between the second electrode 44 and the third electrode 60, a second piezoelectric layer 62 and a fourth electrode 64. The first electrode 40, the first piezoelectric layer 42 and the second electrode 44 have an overlapping area for forming a first resonator 92. The third electrode 60, the second piezoelectric layer 62 and the fourth electrode 64 have an overlapping area for forming a second resonator 94. The bulk acoustic wave device 20 has a resonant frequency and an acoustic wavelength, λ, characteristic of the resonant frequency. The thickness of the first and second piezoelectric layers 42, 62 is substantially equal to λ/2. Furthermore, the device 20 has an acoustic mirror 34 formed between the first electrode 40 and the substrate 30 to reflect acoustic waves back to the first resonator 92. As shown in FIG. 4, a section of the first electrode 40 is exposed for use as a connection point to the signal input end 14 of a balun 10 (see FIG. 5). Similarly, a section of the second electrode 44 is exposed for use as a connection point to the device ground 12. The first resonator 92 and the second resonator 94 have an overlapping area 70, defining an active area of the bulk acoustic wave device 20. The device 20 has a first signal output end 16 and a second signal output end 18.
Ella '886 also discloses a balun for use in applications with lower bandwidth requirements. As shown in FIG. 5, the balun 10 has two identical stacks 21, 21′ of layers, similar to the bulk acoustic wave device 20 of FIG. 4. However, the first electrode 40′ and the third electrode 60′ of the layer stack 21′, and the second electrode 44 and the third electrode 60 of the layer stack 20 are connected to ground 12. In addition, the second electrode 44′ of the layer stack 21′ is connected to the first electrode 40 of the layer stack 21 and is used as the signal input end 14. The top electrode 64 of the layer stack 21 is used as the first signal output end 16, while the top electrode 64′ of the layer stack 21′ is used as the second signal output end 18. With the double-structure, there is no need for the compensation capacitance because the electrodes 60, 60′ below the upper piezoelectric layers 62, 62′ are grounded. This electric shielding effect results in the symmetric impedance for the first and second signal output ends 16, 18. The parasitic capacitance of the dielectric layers 50, 50′ is parallel to the signal input end 14. This parasitic capacitance somewhat degrades the bandwidth of the device but does not harm its symmetry. The cross-connected input electrodes 40, 44′ generate a perfect 180° phase between the acoustic waves in the stack 21 and the stack 21′.
Ella '886 also discloses that the balun 10 can be used as part of a filter that has one unbalanced port and two balanced ports. Two baluns 10 can be coupled to lattice filters 150 to form a duplexer 201 as shown in FIG. 6. In FIG. 6, a phase shifter 242 is used for filter matching. Similarly, two baluns 10 can be coupled to one lattice filter 150 and one ladder filter 250 to form a duplexer 203, as shown in FIG. 7.
It is also possible to form a simple duplexer by using two single-ended ladder filters and a phase shifter, as shown in FIG. 8. As shown in the figure, a single-ended ladder filter 260 is used for Tx and another single-ended ladder filter 262 is used for Rx. However, it usually requires that some inductance components, such as coils, to be connected in series with some of the shunt resonators in the Tx filter order to shift the natural notch to coincide with the Rx frequency. These coils not only cause additional losses in the duplexer, but also create other higher resonance frequencies, further degrading the overall out-of-band attenuation of a single-ended filter. In order to reduce the out-of-band attenuation in the Rx path, it is possible to combine a fully balanced Rx filter with a single-ended Tx filter, as shown in FIG. 9. As shown in FIG. 9, the fully balanced Rx filters 270 are connected to a pair of connected (in series) baluns. The problem with this approach is that any loss associated with the baluns at the antenna port will also cause losses in the Tx path. The Tx path also suffers from the degraded out-of-band due to the inductance.
It is thus advantageous and desirable to provide a simple duplexer that does not have the above-mentioned disadvantageous.